President Barack Obama headed into the final days of his presidency with an unfinished nuclear weapons risk-reduction agenda. But as he prepared to hand off control of the nuclear arsenal to President-elect Donald Trump, his administration announced a further reduction to the U.S. nuclear-weapons stockpile.

In a Jan. 11 speech at the Carnegie Endowment for International Peace, Vice President Joe Biden revealed that the administration had removed 553 reserve warheads from the military stockpile during the past year, reducing the arsenal to 4,018 warheads. Overall, the administration reduced the stockpile by more than 1,000 warheads since taking office in 2009. In periodically making public the warhead numbers over the past eight years, the Obama administration has been more open about the size of the nuclear stockpile than any previous administration.

The decision to retire additional reserve warheads came after Obama and his national security team for months had discussed measures to advance the nuclear risk-reduction goals the president first outlined in his April 2009 address in Prague.